A time domain reflectometer (TDR) is a device that generates an initial electrical signal, couples that signal to a pair of conductors, and observes the electrical waveform at that coupling point. That observed waveform is comprised of an amount of the initial signal and an amount of each signal, if any, that is reflected back from any non-uniformity in the impedance of the conductor pair. The relative amplitudes of the initial signal and the reflected signals are dependent on the relative impedances at each of the signal origination points. The elapsed time (T) between the observation of the initial signal and the observation of the reflections is dependent on the physical distance (D) between those signal points and the velocity of propagation (VOP) for the conductor pair. This VOP is usually between 1/2 and 1 times the speed of light in a vacuum. Since the signal travels from the coupling point to the reflection point and then back, the elapsed time is equal to twice the physical distance divided by the velocity of propagation. T=2D/VOP. In this discussion, T (2D/VOP) is called the "electrical length." One of the common uses of the TDR technique is to determine the distance between the point of connection and a fault in a cable. The fault (shorted wires, broken wire, electrical leakage due to moisture, crushed dielectric, etc.) usually manifests itself as a non-uniformity in the impedance at the point of the fault and is thus observed as a reflected signal. The distance is calculated by the formula D=VOP.times.T/2.
While electrical TDR devices of various characteristics abound, a low-cost device with characteristics suitable for non-professional uses (minimal setup, clarity of readout, absence of special conditions) is not to be found. Methods for solving some of the inherent problems have been described in the literature, but they have not provided a comprehensive low-cost solution.
A first and basic problem is that the low-cost components available for the TDR measurement process have large inherent delays that accumulate to many times the desired resolution of the TDR measurement itself. Additionally, these delays tend to have a large ratio of minimum value to maximum value, and those values are dependent on temperature and power supply. For example, a distance resolution of 5 feet of normal coax cable would require a measurement resolution of 15 nanoseconds. The basic components of a typical low-cost TDR (pulse or step generator, comparator, logic circuit) will have 50 or more nanoseconds of delay, with a possible minimum of 5 nanoseconds. Such a low-cost measurement circuit would ordinarily exhibit a 15 foot distance uncertainty. Without any compensation mechanism, such a TDR could read a 5 foot cable as 20 feet long, or a direct short as a negative 15 feet. Using considerably higher cost components can bring the time uncertainty down to 15 ns from 45 ns, but that still represents an unacceptable 5 foot variance in reading. The various methods to nullify this non-determinate delay as described in the literature add significant cost or complexity to the circuit and have not lent themselves to inclusion in a low-cost TDR product.
A second and related problem is that the non-professional user will often connect the Cable-Under-Test to the TDR via a short set of "convenience wires" that may have impedance characteristics very different from the Cable-Under-Test. Ordinary TDR's require that the Cable-Under-Test be connected directly to the TDR connection point, since the presence of these convenience wires will appear to the TDR to be the first fault in the cable.
A third problem with many TDR systems is the inability thereof to make a valid measurement within a short distance (10-20 feet) of the initial electrical signal point. This distance is related to the internal timing mechanisms and is usually called the "dead zone." While there are methods that reduce or eliminate the dead zone, they are not low-cost.
Some existing TDR products do allow for the calibration of their internal delays and dead zone. Some use internal relays to isolate the test system from the external cable, or require that the user remove all external cables. This invention allows for the calibration of the internal delays with no user action and at a minimal component cost.
The following United States patents are cited as being representative of the prior art: U.S. Pat. No. 5,514,965, issued May 7, 1996; U.S. Pat. No. 5,440,528, issued Aug. 8, 1995; U.S. Pat. No. 5,382,910, issued Jan. 17, 1995; U.S. Pat. No. 4,970,466, issued Nov. 13, 1990; U.S. Pat. No. 4,538,103, issued Aug. 27, 1985; U.S. Pat. No. 4,739,276, issued Apr. 19, 1988; U.S. Pat. No. 5,461,318, issued Oct. 24, 1995.
None of these patents suggest the apparatus and method set forth herein.